This invention relates generally to radiation imaging devices, and more particularly to methods and apparatus for controlling a focal spot of an x-ray tube of an imaging apparatus.
In some known CT imaging system configurations, an x-ray source, such as an x-ray tube, projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. The x-ray beam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated radiation beam received at the detector array is dependent upon the attenuation of an x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam intensity at the detector location. The intensity measurements from all the detectors are acquired separately to produce a transmission profile.
In third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged such that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units” (HU), which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time, a “helical” scan may be performed. To perform a “helical” scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
Reconstruction algorithms for helical scanning typically use helical weighing algorithms that weight the collected data as a function of view angle and detector channel index. Specifically, prior to a filtered backprojection process, the data is weighted according to a helical weighing factor, which is a function of both the gantry angle and detector angle. The weighted data is then processed to generate CT numbers and to construct an image that corresponds to a two-dimensional slice taken through the object.
To further reduce the total acquisition time, multi-slice CT has been introduced. In multi-slice CT, multiple rows of projection data are acquired simultaneously at any time instant. When combined with helical scan mode, the system generates a single helix of cone beam projection data. Similar to the single slice helical, weighting scheme, a method can be derived to multiply the weight with the projection data prior to the filtered backprojection algorithm.
The focal spot size of x-ray tubes used in CT imaging systems can vary substantially as the mA and kV values change over a wide range. X-ray tubes are generally required to produce currents varying from a few mA up to maximum currents near 1 Amp. Due to space charge forces on the electron beam, focal spot size can vary substantial over the entire mA range, as well as the kV range. As a result, artifacts can occur in images produced by a CT imaging system utilizing such tubes. For known prior art CT imaging systems, there is no way to control this variation other than use an an x-ray tube designed to keep the focal spot size within a given range. If the designed range is too wide, image quality, tube performance, or both may be degraded. In particular, as CT imaging systems are being built with increasingly wide detectors and thin slices, variations in spot size with mA is causing increasingly noticeable artifacts in outer rows of volume computed tomographic (VCT) images.
X-ray tubes having dynamic focal spot control have recently become available. These x-ray tubes use sets of control plates near the filament of the tube to shape and deflect the electron beam or a dynamic magnetic field, and thus the x-ray beam. These tubes now make it possible to dial in a given focal spot size by sending appropriate control (bias) voltages to the cathode of the tube or magnetic field controls signals to an magnet.